Rechargeable batteries with non-aqueous electrolytes, represented by lithium-ion secondary batteries, attract attention as batteries for electric vehicles and energy storage because the batteries have high specific energy density. The electric vehicle using lithium-ion secondary battery includes a zero-emission electric vehicle which does not mount an engine, a hybrid electric vehicle which mounts both an engine and a secondary battery and, moreover, a plug-in hybrid electric vehicle which is directly charged from a system power source. An application as a stationary electric power storage system is also expected, which supplies electric power in emergency when an electric power system is shut off.
For these various applications, high capacity density and excellent durability are required for the lithium-ion secondary battery. More specifically, the lithium-ion secondary battery is required which has a high charge capacity, low falling rate of a chargeable capacity even when environmental temperature becomes high, and high capacity retention for long periods. Particularly, large electric energy charged in one-time charge is primarily required for the lithium-ion secondary battery for the electric vehicle. As one of the solutions, an anode material having a high capacity is required. Storage property and cycle life are secondary important required performance at high-temperature environment over 60° C. which is caused by radiant heat from a road surface or heat conduction from inside of a vehicle.
Many materials are known as carbonaceous anode material related to high capacity density. Among them, an art in which rapid voltage reduction at the end stage of discharge is avoided and an art which provides a high capacity battery by mixing a plurality of carbon materials are disclosed.
Japanese Patent No. 3068712 describes an invention in which high and flat discharge potential can be maintained by using an anode made by mixing graphite to 5 to 30% by volume of other carbonaceous material and sudden voltage reduction is prevented by moderately decreasing the voltage between both electrodes when a retention capacity becomes very low during operation of a battery.
Japanese Patent No. 4069784 describes an invention in which a voltage change caused by polarization of the anode at the end stage of discharge is suppressed and a capacity which the anode material itself has is fully utilizable by using a mixed anode made of a graphite-based material and a non-graphite-based material having a higher discharge capacity than the graphite-based material.
Japanese Patent No. 4215202 describes an invention in which capacity deterioration and an increase in internal resistance are suppressed minimally by using an anode made of a graphitizable carbon material which is not fully graphitized and graphite, even when pulse charge-discharge in large current is repeated for long periods, and thereby the longer life can be obtained.
Japanese Patent Application Laid-Open Publication No. 2009-117240 describes an invention which provides a carbon material of an anode of a lithium-ion secondary battery maintaining high capacity density and also having excellent input-output characteristic by using a mixed anode made by graphite and graphite whose surface is modified with a low crystallinity carbon material.
A technique is also proposed which maintains a capacity of a lithium-ion secondary battery by controlling charge conditions.
For example, Japanese Patent Application Laid-Open Publication No. 7-79535 describes an invention in which additional charge is performed when a retention capacity of a battery during charge is detected by using a detection section for the retention capacity of the battery.
Moreover, Japanese Patent Application Laid-Open Publication No. 2000-23388 describes an invention related with a control for increasing the charge capacity in a manner that measures times of charge-discharge or an elapsed time after installing a system and a voltage value and raises charge termination voltage (constant voltage) of a charge circuit to a predetermined voltage value in stages.
A lithium-ion secondary battery for electric vehicles or energy storage may be left at high temperature in a charged state. When the battery is left in such conditions, a decrease in discharge capacity (a charge-discharge capacity) is caused. Among these decreased capacities, there are a capacity which can be recharged and a capacity which cannot be recharged. An object of the present invention is to provide a lithium-ion secondary battery system which can improve the cycle life and the storage property of the lithium-ion secondary battery and can decrease a discharge capacity which cannot be recharged and to provide the lithium-ion secondary battery used in the system.